A multilayer printed circuit board comprises an insulated board of insulated multilayers and electrically conducting layers provided on outer faces of the outer insulated layers and on both faces of the inner insulated layers. Such a multilayer printed circuit board serves to save spaces for printed circuits because the printed circuits are provided on both faces of the inner insulated layer or layers as well as on the outer faces of the outer insulated layers.
Of late, such multilayer printed circuit boards used when circuits are to be formed and various devices are to be mounted on the boards are of a size which is larger than a practical size in which the multilayer printed circuit boards are provided in the electronic instruments. Thus, the multilayer printed circuit boards have to be divided by any conventional dividing device, such as a puncher or a router, into a plurality of board portions or pieces having such a practical size in a finishing step.
In one prior art reference, the insulated board is formed of glass fiber woven cloth layers impregnated with thermosetting resin such as epoxy resin. However, a die of the puncher tends to be easily damaged because of hard glass fiber woven cloth layers. This prevents the multilayer printed circuit board from being inexpensively provided by mass-production. The multilayer printed circuit board is impossible to be punched as the thickness of the board becomes larger. It impractically takes much time for the multilayer printed circuit board to be divided by the router.
In another prior art reference, the insulated board is formed of glass fiber nonwoven cloth layers impregnated with thermosetting resin such as epoxy resin, as disclosed by Japanese Patent Application Laying-Open No. 117,883/1986. Such an insulated board can be easily divided because glass fiber nonwoven cloth layers are so soft as to never damage the die of the puncher. However, since the multilayer printed circuit board comprises only glass fiber nonwoven cloth layers impregnated with thermosetting resin, it undesirably has lower physical strength.
It will be proposed that the multilayer printed circuit board of working size formed of glass fiber woven cloth layers impregnated with thermosetting resin has cutting grooves which are to be used for breaking the board along the grooves whereby it can be divided into the board portions or pieces of practical or mounting size. However, it will take much time for the board to be broken because of hard glass fiber woven cloth layers. In addition thereto, the end faces formed by breaking the board tend to inconveniently have fins or thorns of glass fiber appearing thereon.